AIRBORNE MAGNETIC METHOD: SPECIAL FEATURES AND...

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Aerogeophysics in Finland 1972–2004:Methods, System Characteristics and Applications,edited by Meri-Liisa Airo.Geological Survey of Finland, Special Paper 39, 77–102, 2005.

AIRBORNE MAGNETIC METHOD:SPECIAL FEATURES AND REVIEW ON APPLICATIONS

byJ.V. Korhonen

Korhonen, J.V. 2005. Airborne Magnetic Method: Special Features and

77–102.This article describes the main characteristics of the aeromagnetic com-

ponent of the low altitude survey system of the GTK and discusses futurestrategies in magnetic data reduction. A second part outlines opportunitiesfor the use of the magnetic survey data. The system was initiated in 1972 andhas been used basically the same way until today. Special aeromagneticfeatures of this second National aerogeophysical programme are as follows:1) The system was aimed at refining earlier nation-wide measurements at150 altitude. Hence, it became a low altitude survey (30m) that wasequipped with a transverse horizontal gradiometer for improved resolutionbetween survey lines (the first operational system globally). 2) No tie lineswere used, because the magnetic sources were too close. 3) Transient andsecular corrections were planned to reduce the data to a single event of time(1965.0), hence allowing a free choice of anomaly definition (DGRF 1965.0was used) and offering an easy opportunity for a global contribution. Thefollowing features were developed to facilitate the use of the data since1980: 4) Grey-tone anomaly display was designed for visual interpretationof the maps. 5) Supplementary nation-wide petrophysical mapping pro-vided a link between magnetic anomalies and geological characteristics ofthe sources. 6) Supplementary international data reduction and exchangebetween nearby areas supported regional and crustal scale understanding ofthe sources. Strategically, high-quality results and easy access to datacaused user demand and funding to extend the programme to the wholecountry, although a minor part only was originally planned for refinement.

Key words (GeoRef Thesaurus, AGI): geophysical surveys, geophysicalmethods, airborne methods, magnetic methods, magnetic anomalies, mag-netic field, magnetic survey maps, petrophysics, Finland

Juha V. Korhonen, Geological Survey of Finland, P.O. Box 96,FI-02151 Espoo, Finland

E-mail: [email protected]

BACKGROUND

Between 1951 and 1972, the Geological Survey ofFinland (then GTL, now GTK) carried out the firstnational aerogeophysical mapping programme at anominal altitude of 150 m above terrain (high alti-tude survey). The spatial variation of total magneticfield was measured by a flux-gate magnetometer

along traverses of separation 400 m on land and 500m in the coastal waters and the Finnish economiczone. The flight-time variation of the Earth’s mag-netic field was recorded at a local base station. Datawas processed by analogue methods to hand-drawnanomaly contours of floating base level. In 1968–69

Review on Applications. Geological Survey of Finland, Special Paper 39,

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Geological Survey of Finland, Special Paper 39J.V. Korhonen

the absolute base levels were determined by protonmagnetometer profiles, measured at 40-km intervalsin a NS-direction for the whole country. Since thenthe anomalies have been presented as graphicallyand numerically compiled IGRF-65 anomalies(Puranen & Kahma 1949, Marmo & Puranen 1966,1990, Korhonen 1980, 1991, Ketola 1986, Peltonie-mi 2005, this volume, Fig. 1). The measurementswere complete by 1972 and the map drafting by1980. The aeromagnetic maps were considered high-ly useful in mineral prospecting and bedrock map-ping. To facilitate interpretation and further add tothe value of geophysical maps, an assortment ofpetrophysical measurements from lithological sam-ples was introduced in GTL (Puranen et al. 1968). Anational petrophysical archive was established at1972 (Puranen 1989).

Originally, a low altitude survey of one third ofFinland was presented as an alternative for the high

altitude survey of whole country (Puranen & Kah-ma 1949). Finally, parallel to the high altitude pro-gram, the GTL and mining companies made supple-mentary low altitude surveys to serve mineral pros-pecting in the 1960’s. The Outokumpu Company de-veloped a computer-based processing system for itsdigital airborne data since 1968, having both higherquality and drafting speed of geophysical maps (Ke-tola et al. 1972). At the GTL a computer was appliedto transform the Decca co-ordinates of high altitudeflights over the sea to rectangular co-ordinates. Tostudy alternatives for future geophysical mapping,numerical filtering tests were done for digitised highaltitude data. Although some more details could befound by applying high pass type filters, computerenhancement of high altitude data could not replacemore detailed measurements in prospecting (Korho-nen 1970).

Fig. 1. Aeromagnetic Map, IGRF-65 Anomalies of High Altitude (150m) Survey, Sheet 3723, Peurasuvanto, Original Scale 1:100 000. Red colours100, 300 500, 700 nT, blue colours –100, –300, –500, –700 nT. Sheet 3723 11, Kuusi-Lomavaarat, marked as a grey rectancle.

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Geological Survey of Finland, Special Paper 39Airborne Magnetic Method: Special Features and Review on Applications

LOW ALTITUDE MAPPING

In 1972 GTL started a second national airbornegeophysical mapping programme at a nominal alti-tude of 30 m above the terrain. Track separation was200 m. The programme was aimed for prospectingof sulphide and U-Th ore deposits. Hence the re-quirements of electromagnetic and radiometricmeasurements were dominating in determining theflight parameters and the mounting of the instru-ments at the aircraft. (Peltoniemi 1982, 2005, thisvolume). The menu was completed with two protonmagnetometers, one in the aeroplane (tail beam) andanother at the magnetic base station. The aim wasthat the mutual consistency of survey lines would beassured by tie lines, flown 5 km apart from each oth-er. The system was based on digital data registration,data processing and map drafting. Original and proc-essed digital data was aimed to store systematically

for future use. An example of a resulting 1:20 000 -scale map is shown in Figure 2.

A visual comparison of combined effects of sam-pling and processing of magnetic component at bothmapping programmes is presented in Figures 3a–b.Gradients of the sets are made comparable by con-tinuing the low altitude data upwards 100 m. Thesampling rate of the high altitude data set is c. 20effective stations/km2. A combined effect of coarsersampling and manual map drafting has permanentlyleft out some details from the map. The samplingrate of low altitude data set is 400 stations/km2. Dig-ital processing may suppress or emphasise detailsupon needs. Even as smoothed by upward continua-tion the low altitude set provides a wealth of newdetails to be interpreted geologically.

Fig. 2. Aeromagnetic Map, Absolute Total Intensity of Low Altitude (30m) Survey, Sheet 3723 11, Kuusi-Lomavaara, Original Scale 1:20 000.Coloured by DGRF-70, red +500 nT, blue –500 nT. A test print 1974.

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Fig. 3a. Aeromagnetic Map, Total Intensity, IGRF-65 Anomaly of High Altitude Survey (150m), Sheet 3714, Sodankylä. Original Scale1:100 000. A reproduction of coloured IGRF-65 anomalies of Finland in hand drafted grey scale levels at 200 nT intervals.

Fig. 3b. Aeromagnetic Map, Total intensity, DGRF-65 anomaly of Low Altitude Survey (30m), continued upwards 100m. Sheet 3714, So-dankylä, Original Scale 1:100 000. Digital grey scales by film plotter at c. 40 nT intervals.

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BASIC PLANNING OF THE LOW ALTITUDE FLIGHTS

Changing from the old high altitude measurementsto the new, low-altitude programme at GTL tookplace contemporarily with an administrative transi-tion in which GTL obtained a new Director Gener-al, Dr Herman Stigzelius, a former inspector ofmines at the Ministry of Trade an Industry, the Min-istry hosting GTL. It became policy to initiate anddevelop automatic data processing (ADP) and geo-chemical mapping of the soil at the GTL.

A working group was established to outline theneeds of ADP and, together with the State Comput-ing Centre, prepare a development plan for the years1972–1976 (Puranen and Korhonen 1970). Freelytranslated, the plan describes aerogeophysical meas-urements as follows:

“The most important goal of the aerogeophysicallow altitude mapping will be producing series ofaerogeophysical equal anomaly, profile and interpre-tation maps for the needs of mineral prospecting andgeological mapping. The observations will be madeat an aeroplane or helicopter, transformed to digitalformat and stored on a magnetic tape. The data willbe checked and divided in files organised in nation-al map sheet divisions. The values will be used tocomputer-draw equal-anomaly and profile maps.These maps will be interpreted both by interactiveand semi-automatic methods and will establish dif-ferent series of interpretation maps.”

“Planning of the project will be started in 1971 andthe first equal-anomaly and profile maps are to bemade by 1973. Thereafter planning of interpretation

systems will start. The final year of the project can-not be fixed now because its contents will be trans-formed depending on the registration and interpre-tation methods developed.”

“The measurements can be processed more effec-tively and for varying purposes by computer ratherthan by traditional methods. Producing various in-terpretation map series is, in fact, impossible with-out ADP, because the number of staff and costswould otherwise be too high. The project will in-crease the reliability and speed of geophysical inter-pretation. As compared to the present system, the re-sults will be essentially more useful in mineral pros-pecting and geological studies.”

Afterwards we saw that what happened closely fol-lowed this outline. Special features of the aeromag-netic system were developed to solve problems en-countered in the surveying. Development of thesemethods still continues. Of the other tasks of theADP plan, the geochemical mapping and computereducation of the staff started immediately. Some ob-jectives were understood to be useful in the future,but without any given time schedule. Most of themwere accomplished in due time. Finally a pre-Qua-ternary geological map was printed by ADP-meth-ods (Silvennoinen et al. 1988). In the beginning ADPat the GTL was specialist’s work done by mainframe computers. Gradually ADP became every-body’s tool based on commercially available soft-ware and without major programming and compu-ter capacity limitations.

DATA PROCESSING

Alternatives were studied to buy data processingsystems from GeoMetrics or from the State Comput-ing Centre and some of the computer plotting ofmaps would be done at the GTL. Available compu-ter and data media capacities were determinative fac-tors in all system planning. Finally, a disc-operatedHP2100 computer was bought for time-sharing day-time and batch processing of magnetic maps and tillgeochemical data outside of office hours. Imageswere made using an electrostatic dot-matrix plotter(Versatec). Radiometric and electromagnetic meas-urements were processed separately on a UNIVAC1108 of the Ministry of Education. At the end of1974, an HP3000 series II main frame computer withan advanced file management system was pur-

chased. Later it was updated to a Series III and an-other similar one was bought. Maps were plottedwith two pen plotters (Calcomp) that were regularlyupdated. In 1981, processing was moved to a Digit-al VAX 11-series computer, where all methods weremerged under disk file management in 1984.

The magnetic component was processed as se-quential files from one magnetic tape to another.Simple metadata of path, including information onflight, profile, program version and date was includ-ed as a header of each tape file and further summa-rised as catalogues. The job decks including param-eters were stored on library tapes. The main phasesof production of basic equal anomaly maps 1:20 000were: 1) picking data from data logger tape and

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checking format, 2) finding sporadic values and datagaps, 3) correcting for these or rejecting unreliableparts, 4) correcting for short and long term variationof the Earth’s magnetic field and for fields causedby the aeroplane, 5) merging co-ordinates, 6) com-puting horizontal gradient, 7) interpolating to a 50m x 50 m grid, 8) formatting grid data for map draft-ing, 9) computer drafting equal-anomaly map filesand 10) plotting these on a transparency off line.

Further stages of map production were made spe-cifically for interpretation. Computer processingtook place between direct access disk files and theirmetadata headers as follows: 11) Combining the1:20 000 scale map grids to 100 000 scale map grids,12) subtracting the DGRF-65 normal field to obtainanomalies, 13) converting and scaling anomalies to8-bit grey tone files, 14) plotting these off line withan Optronix 1000 film plotter of the State TechnicalResearch Centre, 15) drafting the films on repro-media at J.Ferin Co, and 16) storing data on librarytapes and book-keeping of these products.

Programs were written on FORTRAN IV forHP3000 MPE-operating system. Main part of thesoftware was designed and tested at the processinggroup. Thereafter, the bulk of the work was to pre-pare the jobs, let the computers do the processing,make intermediate checks and interact when manu-al work was necessary. An important part in longterm was to obtain and analyse magnetic control in-

formation from field measurements to calculate bestpossible control parameters to get consistent datasets from one area and year to another.

By the end of 1980’s the GTK had become one ofthe largest producers of graphic data processing inFinland. As usual in the 1970’s, the need for compu-ter capacity increased more rapidly than new mainframe capacity could be bought. To save operatedCPU-time and tape station resources, and compilethe maps rapidly the main part of the processing ofmagnetic maps was designed for running the jobs aslong batches outside office hours. By two batch-processing computers and two pen plotters, 24 mapsheets could be completed in one weekend insteadof the two months required for parallel processingwith other activities. The base station data were pre-processed by time-sharing in office time.

Initially, Juha Korhonen made programs and proc-essed data to maps, assisted by Tuula Laine for basestation data. Later on, Meri-Liisa Airo took respon-sibility of the processing. Co-ordinates were ob-tained from the processing group for radiometricdata. Maps were monthly released to the public. Fi-nally, 88 magnetic tape series of 1972–1980 dataproducts and corresponding metadata archives wereforwarded to the processing group at the new VAXsystem. The later stages of magnetic processing havebeen described by Hautaniemi et al. 2005 (thisvolume).

SPECIAL FEATURES OF REGISTRATION OF THE MAGNETIC FIELD

Short term consistency: The Auroral zone is amajor source of geomagnetic disturbance in Finland,especially in the north. Hence, lateral variation oftemporal changes of magnetic field was consideredwhen collecting reference information for magneticsurveys. The short-term magnetic variation was cor-rected using data registered at a magnetic base sta-tion that was installed in the survey area (Fig. 4). Thebase station was taken to represent field variationwithin a radius of about 30 km, taking care that nomeasurements are made during magnetically dis-

turbed times. Allowed magnetic variation parameterswere predefined for the field crew to check the mag-netic ‘weather’ from a monitor station at the airbaseprior to each flight. More recently, forecasts of mag-netic weather by solar-terrestrial observatories havebeen used as well.

Tie lines: Although the programme was startedwith measurement of magnetic tie lines, this prac-tice was stopped after being shown to be useless. Theaccuracy of xyz-position at tie knots was too lowwhen compared with the sharp variation of magnet-

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ic anomalies due to close distance to the nearestmagnetic sources. Careful corrections for base sta-tion data and directional aeroplane effects gave moreaccurate results. Hence, by skipping the tie lines, ca-pacity reserved for profile measurements could besaved. Savings were used to increase the annual areasurveyed by five per cent, equalling to one produc-tion year in a twenty years period.

Long term consistency: The aim was to producea nation wide map set in which the levels of mag-netic total intensity would fit together, independent-ly of time and of registration, and further be reliablymerged with any other corresponding data set inNW-Europe or globally. Hence the main field partof the measured total field was corrected to corre-spond to epoch 1965.0. The correction was based onsecular magnetic variation at nearby geomagneticobservatories and tied to the level of the magneticbase station as described in appendix 1 (Fig. 5). Itwas supposed that the anomaly component doesn’tchange considerably upon changes in main geomag-netic field, and hence can be neglected.

Line spacing: It was known already at the plan-ning stage that, at an altitude of 30 m above theground and using line spacing of 200 m, the sharp-est, near-ground parts of the anomalies would bepoorly represented across the lines. Nothing couldbe done about this, however, because of economicreasons and planned schedule of the refinement pro-gramme. In 1975, after facing this fact on maps inpractice, sampling was improved by installing twomagnetometers, one on each wingtip, instead of justone at a wingtip or on the tail, as in 1972–74. Thisdouble profile configuration became the first opera-tional horizontal transverse gradiometer system glo-bally (Figs. 6a–b).

Anomalies: The basic result quantity from themagnetic measurements is magnetic total field re-duced to 1965.0 (absolute magnetic total intensity).This gradually increases from south to north from50000 to 53000 nT over the Finnish territory, mak-ing it difficult to colour the maps with a single scaleand to interpret anomalies in regional terms. Hence,for combining survey-areas and facilitating interpre-tation of crustal sources of anomalies, a normal field,defined as DGRF-65, was subtracted from the abso-lute total intensity (IAGA 2003, Fig. 7a–c). Proc-essed in this way, via absolute total intensity, anom-alies of different registration years fitted togetherwell. The remaining regional anomaly range wastypically from –400 to +800 nT. However, longwavelength components lower than or equal to or-der and degree ten in global spherical harmonic ex-

Fig. 4. Geomagnetic total field variation during flight 60 in 1974 inTörmäsjärvi area (Lat=66.0). From bottom up: a) Nurmijärvi geomag-netic observatory (Lat=60.5), b) base station at Oulunsalo airport(Lat=65.0), c) Sodankylä geomagnetic observatory (Lat=67.3).

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Geomagnetic Drift at Nurmijärvi and Sodankylä

(1965.0 set to zero)

-400

-200

0

200

400

600

800

1000

1950 1960 1970 1980 1990 2000

Year (AD)

Fre

l (n

T)

Nur

Sod

Diff (nT/a)

2nd diff(0.1 nT/a2)

Fig. 5. Relative geomagnetic change in total field at and between Nurmijärvi and Sodankylä geomagnetic observatories 1953–2003. Averages at thereference year 1965.0 are set to zero.

Fig. 6a. Transverse horizontal gradiometer installed at DC-3 (1975–1980 surveys). Photo: GTK archives

Fig. 6b. First and second transverse horizontal gradiometers installed atDeHavilland Twin Otter (1984–1988 surveys). Photo: GTK archives

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a) b)

c)

Fig. 7–c. Magnetic High Altitude Survey (150 m) anomalies of Finland in different scales, red positive, yellow zero, blue negative, grey positivehorizontal gradient. a) total field in total field range (zero 51 500 nT), b) DGRF-65 anomaly in total field range (–1500 … 2500 nT), DGRF-65anomaly in anomaly range (–400 + 700 nT).

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pression were cut off in this anomaly definition. Toobtain continental and global scale anomalies oflonger wavelength than 2600 km some other normalfield definitions must be used. In this data set anydigitally defined, GIS-based normal field is easy toapply because the absolute total intensity data is re-tained, the secular variation data from geomagneticobservatories is available and the present normalfield grid and its definition coefficients are known.

Graphic display: At the first stage, the absolutetotal intensity was represented as equal anomalymaps coloured with reference to DGRF-65 value atthe centre of each 1:20 000 scale sheet). Besidestheir use in geological studies these maps were orig-inally planned to establish an analogue safe archiveto numerical grid values of absolute total field in50nT accuracy class, in case of eventual loss of dig-ital data. For interpretation, these maps were com-bined as DGRF-65 anomalies in grey-scale (greytone maps) at a scale of 1:100 000 from 1980 (Kor-honen 1983) (Figs. 8, 9a). The latter presentationbecame popular because of its good visual proper-ties and economic price. More recently users havestarted to prepare coloured and enhanced maps indi-vidually for each problem and area using databasefor low altitude airborne geophysics of the GTK asthe source of the data.

National coverage: Although the program wasstarted to re-fly key areas for mineral prospectingonly, the concept was soon extended to cover thewhole country. The main reason for this was that thelow altitude survey brought in new useful detail forgeological studies, independent of region and pur-pose. The maps were considered so useful to region-

Fig 8. Magnetic High Altitude Survey (150 m) IGRF-65 anomalies ingrey scales (50 nT interval, dark positive), original scale 1:2 000 000.The first grey tone map at the GTK in 1980, a by product of printingthe Magnetic Anomaly Map of Finland (Korhonen 1980)

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Fig. 9a. Aeromagnetic map, total intensity of DGRF-65 anomaly, 30 m above ground. Sheet 2741, Sirkka. Standard grey tone map, based ontotal intensity an its horizontal gradient. Original scale 1:100 000.

Fig. 9b. Aeromagnetic map, horizontal gradient of total intensity, 30 m above ground. Sheet 2741, Sirkka. Special grey tone map, presentingintensity of horizontal gradients that were used to compile map 9a. Original scale 1:100 000.

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Fig. 10a. Digital combination of high and low altitude aeromagnetic grids of GTK, in grey tones at c. 40 nT intervals. Map Sheet 33, Iisalmi. Heightof the map area is 120 km. Major squares represent 1 km x 1 km grid. Edited from original scale 1:400 000.

al geological activities that the availability of newmaps could influence on scheduling a new geologi-cal project. In turn, the need for countrywide over-views arose (Fig. 10a–b).

Regional and Global coverage: To facilitate in-terpretation of the long wavelength anomalies, GTKexchanged a 1 km x 1 km grid of DGRF-65 anoma-

lies of high altitude data with the neighbouring coun-tries to have access up to 200 km outside the bor-ders of Finland. In these compilation processes sim-ilar schemes of reduction as applied at GTK havebeen shown to be useful so far. Independently col-lected and reduced national matrices fitted with eachother without any major forcing, like in examples of

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Fig. 10b. Aeromagnetic map, Central Finland, DGRF-65 anomalies of total intensity in grey tones at c. 40 nT intervals. The map was prepared as acollage of digitally combined of low and high altitude grids of 1:400 000 map sheets (e.g. Fig. 10a). Compiled by J.Korhonen and M-L. Airo in1984. Presented in exposition of GTK at 54th SEG meeting in Atlanta 1984. Height of the map area is 480 km. Edited from original scale1:400 000.

Figures 11a–b. The Finnish anomaly field would befurther joined together with a global database col-lected by IAGA on a 5 km x 5 km grid (Korhonen1997).

Petrophysics: GTK has covered whole country bypetrophysical measurements on hand specimens anddrill cores. The national petrophysical database con-

sists of bulk densities and basic magnetic propertiesof 131 000 samples. The purpose is to rapidly pro-vide values for first approximations of petrophysi-cal properties in geophysical modeling and geologi-cal interpretation of anomalies over any area of Fin-land.

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Fig. 11a. Aeromagnetic Anomaly Map, Northern Fennoscandia, Total Intensity Referred to DGRF-65. Original scale 1:1 000 000 (Korhonen etal. 1986).

Fig. 11b. Magnetic Anomaly Map, North Finland – Kola, DGRF-65 Anomaly of Total Field, 500 m above terrain. Original scale 1:1 000 000(Korhonen et al. 2001a).

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TRANSVERSE HORIZONTAL GRADIOMETER

The characteristics of electromagnetic and radio-metric measurements defined the flight altitude to beas low as possible. The average distance to magnet-ic sources in low altitude data was estimated by thenominal flight altitude 30 m, added with mean soilthickness of 4 m in Finland. Hence the minimumhalf width of anomalies caused by geological nearsurface sources was supposed to be less than 25 m.It was well understood that a flight line configura-tion of 200 m track-separation greatly under-samplessuch features across the profiles (Korhonen 1970).A 50m track separation would have been necessaryto adequately sample the anomalies in both dimen-sions. However, there were no economic possibili-ties to do this at all. The programme was started withone magnetometer in the tail of a De Havilland TwinOtter. Map drafting procedures produced oblique lin-ear anomalies as ‘chains of pearls’. In this situationit was suggested again to fly with closer line spac-ing, but the proposal was considered impossible toaccept. In fact, alternatives were studied so as to es-sentially lower the total costs, even by cancellingpart of the new programme. However, the new mag-netic maps were considered to be essentially moreuseful than the previous high altitude maps, andcheaper per unit area than making ground basedmeasurements. Because the survey seemed to beworth its price, the Ministry of Trade and Industryagreed to continue, and later even to extend the pro-gramme.

Meanwhile, the Twin Otter had crashed on a pas-senger flight in wintertime, and low altitude surveyswere continued using a DC3. On the new platformthe electromagnetic system was co-axial and themagnetometer was moved to the left wingtip. Theinnovation was to mount a second magnetometer onthe right wingtip, hence providing a double profileof track separation 24.5m at a single price. This sec-ond magnetometer was built from system spareparts. With the skills of the instrumentation team itworked with minimal problem from the first testflight onwards (Fig. 6a). Due to close distance to thesources this simple measurement was capable ofidentifying maximums and minima between flightlines by calculated horizontal gradients and deter-mining directions of equal anomalies at the flightlines (Figs. 12a–b).

Fig. 12a. Horizontal gradient vectors calculated from DC-3 data 1978.Flight lines are in NS-direction and 200 m apart from each other. Thedistance between co-ordinate lines is 2 km.

Fig. 12b. Directions of equal anomalies at maximum gradients (lines),total field maximums (circles) and minima (crosses) calculated fromDC-3 data 1978. Flight lines are in NS-direction and 200 m apart fromeach other. The distance between co-ordinate lines is 2 km.

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Directional aeroplane corrections were calculatedfor both magnetometers by data registered at theEmäsalo magnetic rose site that was measured fromthe air a few times per year. An interpolation algo-rithm for gridding by gradients was adapted from themanual of the contouring program GPCP II (Cal-comp 1972). From the pair of two wingtip profiles,both horizontal components of gradient were calcu-lated at each data station and were used to linearlyextrapolate the total field to the grid point. Estimatesfrom various stations were weighted inversely bytheir square distance. Data from three closest pro-

files in a radius of 400 m were used in the calcula-tion. The interpolation scheme produced more iso-tropic and geologically looking anomaly patternsfrom two magnetometer data than from one magne-tometer only (Figs. 13a–b.) It placed anomaly highsand lows mostly between flight lines, although it didoverestimate values in high gradient cases. This lat-ter could have been avoided by a more complicatedversion of the procedure. However, the interpolationwas kept simple to compile the maps in a reasona-ble time.

Fig 13a. Magnetic total intensity, equal anomalies at 50 nT intervals, compilation of left magnetometer,Sheet 3344 07, Hiidenvaara. Coordinate lines are 2 km from each other. North is to the left.

Fig 13b. Magnetic total intensity, equal anomalies at 50 nT intervals, compilation of both magnetometers,Sheet 3344 07, Hiidenvaara. Coordinates are as in Fig 13a.

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To test the method, the details of the gradient basedtotal intensity maps were compared with bothground measurements of GTK and low altitude flightsurvey of the Outokumpu Company. In Korsnäsarea, the gradient-based system on track separationof 200 m (400 stations/km2) indicated ground anom-alies closer than a single magnetometer flight oftrack separation 125 m (320 stations/km2), both hav-ing an effective along track station spacing of 25 m.The grid values created by the gradient interpolationscheme were better nearer the flight lines than in midareas, as might be expected.

Development ideas included using the gradientvectors in identifying different source types, and as-sisting in automated interpretation. Second trans-verse gradient could help in interpolating grids of thefirst gradient for interpretation (Figs. 9a–b). Longi-tudinal gradient measurement could help in correct-ing for short-term magnetic variation near crustal-

scale conductors and along long profiles. Further-more, a tensor gradient configuration was an attrac-tive idea, but far beyond the possibilities of a nation-al mapping team with underlined practical goals(Korhonen 1984, 1985). A three magnetometer trans-verse system was tested in the Twin Otter 1984–1988 (Fig. 6b). It was abandoned because of an una-voidably high noise level from electromagneticsources in one of the sensors.

More recently, old survey areas have been re-cov-ered to complete a more dense line spacing down to50 m for mineral prospecting. In fact, a sufficientlydense data sampling is one of the basic requirementsto distinguish between potential field sources ex-posed at the bedrock relief surface and deeper unex-posed sources above the drilling depth of normalprospecting (c. 500 m). A few uncovered map sheetsof the programme on land area are planned to sur-vey in 2005–2006.

GEOLOGICAL APPLICATIONS OF MAGNETIC MAPS

In the 1970’s, geologists used magnetic anomalymaps at scales 1:20 000 (10 km x 10 km squares)and 1:50 000 (collected to 20 km x 30 km rectan-gles) where total intensity was presented as 50 nTequal-anomaly lines. The former were used to visu-ally interpret anomaly sources and geological struc-tures locally, the latter to see overall geological ele-ments of a survey area. Maps were drafted with 10or 2 nT contour line intervals when more detail wasrequired. Starting in 1981, map-sheet grids weremerged to 1:100 000 scale matrices (40 km x 30 km)on a 50 m x 50 m grid, DGRF-65 anomalies werecalculated and grids were drafted via film to grey-tone repro-paper or transparency. Altogether 131map sheets were released up to 1985. Following thegeneral trends of computer data processing, thedrafting of special maps was transferred to projectgroups and finally to the GIS-groups of the Survey.

The maps and grids have been used as one of thebasic materials in bedrock mapping, mineral re-source assessment, mineral prospecting and studiesof groundwater reservoirs, both in the public and theprivate sectors. Interpretation of maps, both visuallyand numerically, was normal but practice varied inorganisations using the data. (e.g. Aarnisalo et al.1983, Rekola & Ahokas 1986, Kuosmanen 1988,Säävuori et al. 1991, Ruotoistenmäki 1992, Airo1999, Arkimaa et al. 2000, Pesonen et al. 2000).

A programme to create a total of 342 map sheetsof pre-Quaternary geology at 1:100 000 scale wasinitiated at the Geological Survey in 1946. Aerogeo-physical maps were planned to assist in that workbecause continental Finnish bedrock is for 97 percent covered by Quaternary formations like till, clay,peat bogs and lakes. Since then, 14 maps have beenprepared without aerogeophysical information, 104by using high-altitude maps, and 105 based on lowaltitude data. Quality and outlook differences be-tween these groups clearly exist. Geologically inter-preted magnetic patterns and anomaly boundarieshave been introduced to bedrock maps. Hence, theamount of detail at and continuity of geological for-mations is greater on newer geological maps. Someof the older maps have been revised in connectionwith newer projects. Besides this national mappingprogramme geological maps are made topically, in-dependent of map sheet borders. In these compila-tions the overview characteristics of magnetic mapshave shown to be most useful. The conventional wis-dom of the geologist in their work is that magneticdata is considered to be inferior only to geologicalfield observations.

Aeromagnetic maps have been used in the plan-ning of some major engineering projects in bedrock,like the Päijänne freshwater tunnel (120 km), under-ground fuel storage, nuclear power plants and nu-

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clear waste disposal sites. In addition, the maps es-tablished a reserve of quality background informa-tion in various geology-related investigations, likeenvironmental evaluation, land use planning andgeo-medical studies. They have been especially val-uable in geological-geophysical-geodetic research.Guides and articles on the use of the data have beenpublished mainly in Finnish. (e.g. Marmo & Puranen1966, 1990, Korhonen 1983, 1993, Peltoniemi 1988,Airo 1999).

In 1980, GTK established a team to build back-ground for interpretation of aerogeophysical data. Its

work included conducting a national petrophysicalsampling programme in 1980–1991, compiling aer-omagnetic grey tone maps and multinational mapsets, writing articles for geological use, arrangingworkshops and taking part in geological mappingand research projects. One of its major recommen-dations for future was to build a geophysical crustalmodel for Finland and surrounding area to becomea tool for digital retrieval of previous work and newinterpretation of geophysical data (e.g. Korhonen1992b, 1997, 1999).

PETROPHYSICS

Measurements of physical properties of rocks asan aid to geophysical map interpretation started atGTL in 1953. Systematic measurement of regionalsample sets and drill cores was carried out since1963 (e.g. Puranen et al. 1968). Basic quantitiesmeasured were density, susceptibility, and intensityof remanent magnetisation plus – optionally –vectorcomponents of the latter, thermal dependence of sus-ceptibility, electrical conductivity, IP-effect, porosi-ty, P-velocity and thermal conductivity. The petro-physical database was established in 1973 and con-sists presently of 171 000 data records (Puranen1989, Säävuori & Hänninen 1995). A petrophysicalprogramme covering all of Finland was initiated in1980 (Korhonen et al. 1989, 1993, 1997). The labo-ratory was computerised in 1982 and two new labo-ratories were built at regional offices. Three Finnishuniversities established their own petrophysical lab-oratories. A temporal-spatial summary of Finnish,Norwegian, Swedish and Estonian data of Precam-brian rock properties was presented on maps of theFennoscandian Shield (Korhonen et al. 2002a–b). Amajor part of the digital continental petrophysicaldata globally has been collected in this area (Korho-

nen & Purucker 1999). Application of petrophysicsin interpretation is described e.g. in Airo 2005 (thisvolume).

Local magnetic anomalies are due to the ferrimag-netic population which relative proportion in geolog-ical formations varies regionally from a few per centto almost 100 per cent and is c. 25 per cent on aver-age (Fig. 14a). The bulk density of ferrimagneticrocks varies, corresponding to acid compositions atmajor positive regional anomalies and to basic com-positions at many local anomalies (Fig. 14b). Themapping allows estimation of regional variation inintensity of magnetisation of the ferrimagnetic pop-ulation and total population (Figs. 14c–d). The Q-value varies typically from 0.1 to 20 on sample lev-el and from 0.8 to 8 between major rock types (e.g.Fig. 1 in Korhonen 1993). Scatter diagrams of mag-netic properties indicate typical lithology by densityand mineralogy by ferrimagnetic Q-value for mag-netic anomaly sources (Fig 15a–b). Average magnet-ic properties of major upper crustal units differ, ex-hibiting apparent temporal trends across geologicalhistory of the Shield as indicated by summaries inKorhonen et al. 2002a–b.

95


Fig 14a. Percentage of ferrimagnetic rocks in Finland, Original Scale1:4 000 000. Petrophysical mapping by GTK, 1994.

Fig 14b. Bulk Density of ferrimagnetic rocks in Finland, OriginalScale 1:4 000 000. Petrophysical mapping by GTK, 1994.

Fig. 14c. Magnetization of ferrimagnetic rocks in Finland, OriginalScale 1:4 000 000. Petrophysical mapping by GTK, 1994.

Fig. 14d. Total magnetisation in Finland, Original Scale 1:4 000 000.Petrophysical mapping by GTK, 1994.

96


Svecofennian Domain C

10

100

1000

10000

100000

1000000

2400 2600 2800 3000 3200 3400

Bulk Density kg/m3

Su

scep

tib

ilit

y 1

0E

-6 S

I

Svecofennian Domain C

0,01

0,1

1

10

100

1000

1 10 100 1000 10000 100000

Total magnetization mA/m

Qf

MULTINATIONAL MAPS

Fig. 15a–b. Petrophysical scatter diagrams from database FINPETRO. Primitive arc complex of Central Finland (1.93–1.87 Ga) as defined by Korsman et all. 1997). a) bulk density versus magnetic susceptibility; concentration of ferrimag-netic sources (k>2000) to bulk densities around 2700 kg/m3 corresponding to rocks of intermediate composition (top),b) total magnetisation versus ferrimagnetic Q-value (H=41A/m); concentration of ferrimagnetic sources (J>0.2A/m) toQ-values less than 1, corresponding to coarse grained magnetite (bottom).

High altitude anomalies digitised on 1 km x 1 kmgrid were used as overview material, in data salesand in bi- and multinational data exchange. The re-duction procedure for temporal variation proved tobe adequate to combine national data sets, original-ly compiled independently. Existing geomagnetic

observatory network in the area of participatingcountries at NW Europe was a necessary require-ment to successful compilation. Minor warping hasbeen done locally to adjust data sets to the most re-liable grid.

97


Data versions and corresponding small-scale mapsbased on Finnish Magnetic High Altitude Grid forthe Finnish part include:– FINMAG 00 (1978): Incomplete land IGRF-65

grid.– Ladoga-Bay of Bothnia Zone (Elo et al. 1968).

– FINMAG 01 (1980): Complete land IGRF-65grid, except eastern border zone, part of Baltic Seadata was included.– Magnetic Anomaly Map of Finland 1:2 000 000

(Korhonen 1980)– Magnetic Anomaly Map of Northern Fennos-

candia 1:1 000 000 (Korhonen et al. 1986)– Magnetic Anomaly Map of Finland, Atlas of

Finland, Geology, Map 26a (Korhonen 1992a)1:5 500 000

– Local Magnetic Anomalies of Finland, Atlas ofFinland, Geology, Map 26b (Korhonen 1992a)1:5 500 000

– Magnetic Anomaly Map of the Arctic Area1:5 000 000 (Verhoef et al. 1996)

– FINMAG 02 (1993): Eastern border zone wascompleted by measurements in 1993; completeFinnish DGRF-65 grid of land area and economyzone in the Baltic Sea.

– Magnetic Anomaly Map of Central Fennos-candia 1:1 000 000 (Ruotoistenmäki et al.1996)

– Magnetic Anomaly Map of Europe 1:5 000 000(Wonik et al. 2001)

– FINMAG 03 (2001): DGRF-65 anomalies;BEAR98-levels were applied to anomalies ofeconomy zone; Fits with the surrounding nation-al grids of Norway, Sweden, Russia and Estonia.– Magnetic Anomaly Map of Central Finland

Karelia 1:1 000 000 (Korhonen et al. 2001a)– Magnetic Anomaly Map of North Finland –

Kola 1:1 000 000 (Korhonen et al. 2001b)– Magnetic Anomaly Map of the Fennoscandian

Shield 1:2 000 000 (Korhonen et al. 2002) (Fig.16)

– Magnetic Anomaly Map of Gulf of Finland andsurrounding area 1:1 000 000 (Korhonen et al.in prep)

– FINMAG 04 (2007, in prep): WDMAM-2007anomaly definition for 5 km x 5 km grid extend-ed to 1 km x 1 km grid.– World Digital Magnetic Anomaly Map

1:50 000 000, (IAGA and CGMW, in prep for2007).

Fig. 16. Magnetic Anomaly Map of the Fennoscandian Shield, DGRF-65 Anomaly of Total Field, Anomaly continued upwards 500 m above ground.Original scale 1:2 000 000. Korhonen et al 2002.

98


DISCUSSION

Recently, a global model of geomagnetic variation,the Comprehensive Model (CM), became availablefor reduction of aeromagnetic data (Sabaka et al.2002). In the NW-Europe the CM is based on thesame observatory data as the reduction system ofGTK. Hence a comparison between results obtaina-ble via the CM and from the system of GTK wouldbe useful to understand the accuracy of the reduc-tions in practice, considering that basic assumptionsare valid as defined in Appendix 1.

The map reduction procedure was based on the as-sumption that temporal variation of the Earth’s mainmagnetic field can be traced by and corrected forgeomagnetic observatory variation. This is not com-pletely true in a strict sense, however. A geomagnet-ic observatory records the entire magnetic field at itssite, including the lithospheric anomaly. The chang-es in the core field cause changes in the anomalyfield, immediately in the induced part and with sometime lag in the remanent part. All these are seen as asecular change of the main field, although the lithos-pheric part may be quite local. Changes in the direc-tion of inducing field may change anomaly effectsat observatories more than changes in its intensity.For example, the Sodankylä geomagnetic observa-tory is situated in a regional low north of a majorregional high anomaly, and Nurmijärvi observatoryat the southern slope of a regional high, both beingsusceptible to temporal changes in anomalies.

Although the drift of crustal anomalies is likelyfrom the physical nature of the magnetic field,there’s no definitive scientific evidence that thesechanges have truly occurred at the observatories(McMillan and Thomson 2003). This is why GTKcarried out, in 1998, together with the EURO-PROBE BEAR-project, a country wide reconnais-sance survey based on magnetometer network, forcomparison with Finnish low altitude data since1972. Especially it was intended to see whether itwas possible to detect local temporal components inthe magnetic absolute field, and if so, to estimatehow much this change would contribute to the ob-servatory means and further countrywide anomalylevels over time. It was expected that change since

1972 would be of the order of 10–20 nT at most. Thestudy is to be completed.

Problematic is that the longest wavelength anom-aly components are regularly missing from grids andmaps because of the overlapping bandwidth defini-tion of most normal field models. A spatial model ofthe crustal field sources should be made to under-stand what the smoothest part of the crustal contri-bution may be and what could be its effect on changeof crustal anomalies in time.

The Earth’s magnetic field is rapidly decreasingsince 1000 AD. The quadrupole term energy ofspherical harmonics of IGRF may be equal to the di-polar anomaly term energy 50 years from now (e.g.Gianibelli et al. 2004) should the changes continueat the current rate. It follows that, close to the presentdipolar and quadrupolar anomalies, the intensity anddirection of the magnetic field may change consid-erably. Some time in the future the direction of themain field would reverse. With a change of direc-tion of the Earth’s main field, magnetic measure-ments aimed for global correlation should be pre-sented with reference to the time of observation, notreduced to a common epoch by present methods.Regional compilations could be made by reductionof the data by assumptions of the effective vectoralmagnetisation of lithospheric blocks.

Regional sampling may provide local estimates ofmagnetisation near crustal surface (E.g. Lahtinenand Korhonen 1995) and occasional deeper lithos-pheric values may be obtained from xenoliths but itis impossible to extend sampling to cover the wholelithospheric depth. In fact, necessary lithosphericscale magnetisation and its time dependence can bedetermined by geophysical models of lithosphericunits only, jointly with geomagnetic, gravimetric,seismic, geothermal and geoelectric information. Fi-nally, the modelling groups would end with spatial-ly varying standard lithospheric models, acceptedmore or less globally. The Fennoscandian Shield iscurrently one of the most well covered areas of theglobe for lithospheric modelling and hence wouldbecome one of the type areas both for reduction tech-niques and for characteristics of geophysical sources.

99


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101


APPENDIX

Time dependent reductions for aeromagneticmeasurements

M. Puranen and J.Korhonen18.1.1973 (in Finnish)Geological Survey of FinlandEnglish translation with explanationsby Juha V. Korhonen

Reductions are calculated to tie airborne measure-ment of magnetic total intensity (F

rec) made at time

moment t2

to a fixed time epoch, denoted by t0. The

reduced value is called the absolute value of themagnetic field at that epoch (F

abs(t

0)). The reductions

include both correction for the annual change in theEarths main field (secular variation) within the sur-vey area and correction for the variation in the mag-netic field during the measurements (transient var-iation). The change in the intensity of induced mag-netic anomalies due to change of inducing magneticfield is neglected.

Secular variation is calculated as a difference be-tween observatory magnetic field values (F

obs) at

time epochs of t1

and t0. The difference between the

secular variations in the observatory and in the sur-vey area (dF

sec) is added to the value of observatory

secular variation to obtain the secular variation to beused in reductions for the survey area.

The transient variation is corrected by using mag-netic field values (F

stat), recorded by a fixed magnet-

ic ground station near the survey area. The drift ofthe magnetic field in the aeroplane during the flightis assumed to be the same as the drift in the magnet-ic ground station between a fixed time epoch t

1and

time moment of measurement t2..

In addition a correction will be applied to removethe effect of magnetic field caused by the aeroplane(dF

dir). This varies in flight direction.

The correction formula (1) is the following:

(1) Fabs

(t0) = F

rec(t

2) – (F

obs(t

1)-F

obs(t

0)+dF

sec(t

1,t

0)) –

(Fstat

(t2)-F

stat(t

1)) + dF

dir

Fabs

(t0) absolute magnetic total field intensi-

ty reduced from the moment of re-cording to epoch t

0.

Values for the quantities are calculated as fol-lows:

Frec

(t2) magnetic field value recorded in the

aeroplane (original survey data)

Fobs

(t0) magnetic field average of undis-

turbed days for the epoch of absolutereduction, obtained from the annualreports of the observatory (e.g.1970.5). (1965.0 was used for theFinnish low altitude survey)

Fobs

(t1) average total field for an epoch of

one hour (e.g. 9.00–10.00 UT), cal-culated from H and Z component ob-servatory magnetograms. (Averagesfor several hours were used. Later onthe observatories started to deliverdigital F-data, that was used insteadof magnetograms.)

dFsec

(t1,t

0) difference of secular variation be-

tween observatory and survey area,calculated from secular variation ta-bles or interpolated from maps. (Thiswas calculated from secular varia-tion polynomials provided by the Ge-omagnetic Department of the Finn-ish Meteorological Institute)

Fstat

(t1) magnetic field average calculated

from total field values in the groundstation (The same time epoch, onehour or several hours, was used forall t

1calculations in the same ground

station and survey area)

Fstat

(t2) magnetic total field value at the time

moment t2, interpolated from ground

station measurements (the airbornemeasurements were made at 0.5–0.25 sec intervals and the groundstation recordings at 10–sec inter-vals depending on the instrumenta-tion and survey specifications)

102


dFdir

correction for magnetic field causedby aeroplane in the flight direction ofthe profile (see M.Puranen andL.Kivekäs, 13.12.1972)(fully automatic magnetic correc-tions for aeroplane direction, pitch,roll and yaw was applied since 1994)

t0

time interval (epoch) of a year, e.g.some internationally agreed refer-ence year, denoted by its averagetime expressed in year and one ormore decimals

t1

time interval (epoch) of magnetical-ly silent hour, or several hours, dur-ing the survey, used to tie togethersecular and transient variations, anddenoted by its average time in hourand decimals

t2

time moment of measurement ofmagnetic field value in aeroplane,considered as a sharp point of timeunlike the t

0and t

1that are time peri-

ods

In practice the reductions are made as a computerrun for each of the survey flights. The correctionsare grouped to consist of two terms: the first one isa constant for each profile (F

corr+F

dir) and the second

one depends on time (Fstat

(t2)).

The constant reduction term is calculated as fol-lows:

(2) Fcorr

= +(Fobs

(t1)-F

obs(t

0)+dF

sec(t

1,t

0)) – (F

stat(t

1) –

Fbase

)

Fbase

selected technical level of magneticfield presentation (a value of 50000nT was used for 16-bit computers)

In each measurement point the correction is cal-culated as follows:

(3) Fabs

(t0) – F

base = F

rec(t

2) – (F

corr-dF

dir) – F

stat(t

2)

Fstat

(t2) is interpolated from ground station val-

ues nearest to time t2.

The input of the data can be done as follows:

Data 1. Magnetic tape of pre-checked air-borne magnetic field values and timefor each

Data 2. Correction file, consisting of con-stant correction and directional cor-rections for each profile in Data 1.

Data 3. Magnetic file of pre-checked groundstation records or alternatively a to-tal field file digitized from observa-tory magnetograms and made abso-lute.

Result Magnetic tape containing absolutemagnetic field intensities(together with all other information).

These formulas have been used since 1973 topresent (2005). The level differences indicate an ac-curacy normally better than + 5 nT

AIRBORNE MAGNETIC METHOD: SPECIAL FEATURES AND...

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